Capacitors such as lithium ion capacitor are applied to uses such as auxiliary power source for various types of equipment and regenerative energy storage, including the use for the buffer of wind power generation modules, since they are excellent in power density, safety, and cycle properties, and also have a long life.
A separator for capacitors is generally subjected to heat treatment during the manufacturing process for the purpose of removing water, and it is necessary to increase the processing temperature for improvement of quality and productivity. Moreover, when it is mounted implemented on a circuit board, for example, it needs to pass through a soldering step using the reflow soldering method, and therefore, an enhanced heat resistance is now called for. Further, to provide capacitors with high output, the separators are required to have properties such as low resistance and reduced thickness at the same time.
On the other hand, also in nonaqueous secondary batteries such as lithium ion secondary battery, separators with increased heat resistance, low electrical resistance, and reduced thickness are now called for since there are increasing demands for products with increased capacitance, output, and size, as well as improved safety.
To meet the requirement for increased heat resistance, there has been a proposal of a separator consisting of a heat-resistant layer on one side or both sides of a microporous film which comprises a low melting point resin such as polyolefin (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 2001-23600). However, although such a separator has shutdown property, there are problems such as inability to resist the use in a high temperature atmosphere, which is requited for in batteries with an increased size or a larger number of incorporated cells, and unsuitability for high heat treatment during the battery production process. Moreover, since it is inferior to single-film heat-resistant separators in terms of heat resistance, and tends to cause a short circuit in the end portion due to thermal contraction during abnormal heating, this problem becomes more remarkable with batteries with an increased size, specifically, those with an increased surface. In addition, reduction in thickness is generally difficult because of their laminated structure.
Thus, since aromatic polyamides are substantially free from a melting point, and have high heat resistance, it has been proposed to simply use a nonwoven fabric or paper-like sheet of aromatic polyamide fiber as separator unit (e.g., see Japanese Unexamined Patent Publication (Kokai) Nos. Hei 5-335005, Hei 7-37571 and Hei 7-78608). However, it is difficult to industrially produce uniform nonwoven fabrics or paper-like sheets having a sufficient strength as well as a small thickness of 50 μm or less.
On the other hand, to reduce the resistance of a separator, it is effective to improve the affinity between the separator and an electrolyte, in addition to reducing the thickness and increasing the permeability, and Japanese Unexamined Patent Publication (Kokai) No. 2003-347166 has proposed a separator with affinity developed by carrying out plasma treatment for nonwoven fabrics of aromatic polyamide or the like. However, the plasma treatment method has problems such as rapid deterioration in hydrophilicity over time, inability to improve the surface of pores in the film, and breakage of molecular chains due to plasma irradiation to cause a decrease in the strength of the film.
As mentioned above, there is increasing demands for separators having reduced thickness, high heat resistance, high air permeability and good affinity to electrolytes.
It could therefore be helpful to provide a porous film comprising an aromatic polyamide as a constituent component and having a reduced thickness, excellent heat resistance, high air permeability and good affinity to electrolytes, as well as separators for capacitors and batteries produced therefrom.